The present invention, in some embodiments thereof, relates to isolated polypeptides and polynucleotides, nucleic acid constructs comprising same, transgenic cells comprising same, transgenic plants exogenously expressing same and more particularly, but not exclusively, to methods of using same for increasing abiotic stress tolerance, growth rate, biomass, vigor, yield (e.g., seed yield, oil yield), oil content, fiber yield, fiber quality and/or fertilizer use efficiency (e.g., nitrogen use efficiency) of a plant.
Abiotic stress (ABS; also referred to as “environmental stress”) conditions such as salinity, drought, flood, suboptimal temperature and toxic chemical pollution, cause substantial damage to agricultural plants. Most plants have evolved strategies to protect themselves against these conditions. However, if the severity and duration of the stress conditions are too great, the effects on plant development, growth and yield of most crop plants are profound. Furthermore, most of the crop plants are highly susceptible to abiotic stress and thus necessitate optimal growth conditions for commercial crop yields. Continuous exposure to stress causes major alterations in the plant metabolism which ultimately leads to cell death and consequently yield losses.
The global shortage of water supply is one of the most severe agricultural problems affecting plant growth and crop yield and efforts are made to mitigate the harmful effects of desertification and salinization of the world's arable land. Water deficit is a common component of many plant stresses and occurs in plant cells when the whole plant transpiration rate exceeds the water uptake. In addition to drought, other stresses, such as salinity and low temperature, produce cellular dehydration.
Drought is a gradual phenomenon, which involves periods of abnormally dry weather that persists long enough to produce serious hydrologic imbalances such as crop damage and water supply shortage. In severe cases, drought can last many years and results in devastating effects on agriculture and water supplies. Furthermore, drought is associated with increase susceptibility to various diseases.
For most crop plants, the land regions of the world are too arid. In addition, overuse of available water results in increased loss of agriculturally-usable land (desertification), and increase of salt accumulation in soils adds to the loss of available water in soils.
Salinity, high salt levels, affects one in five hectares of irrigated land. This condition is only expected to worsen, further reducing the availability of arable land and crop production, since none of the top five food crops, i.e., wheat, corn, rice, potatoes, and soybean, can tolerate excessive salt. Detrimental effects of salt on plants result from both water deficit which leads to osmotic stress (similar to drought stress) and the effect of excess sodium ions on critical biochemical processes. As with freezing and drought, high salt causes water deficit; and the presence of high salt makes it difficult for plant roots to extract water from their environment. Soil salinity is thus one of the more important variables that determine whether a plant may thrive. In many parts of the world, sizable land areas are uncultivable due to naturally high soil salinity. Thus, salination of soils that are used for agricultural production is a significant and increasing problem in regions that rely heavily on agriculture, and is worsen by over-utilization, over-fertilization and water shortage, typically caused by climatic change and the demands of increasing population. Salt tolerance is of particular importance early in a plant's lifecycle, since evaporation from the soil surface causes upward water movement, and salt accumulates in the upper soil layer where the seeds are placed. On the other hand, germination normally takes place at a salt concentration which is higher than the mean salt level in the whole soil profile.
Germination of many crops is sensitive to temperature. A gene that would enhance germination in hot conditions would be useful for crops that are planted late in the season or in hot climates. In addition, seedlings and mature plants that are exposed to excess heat may experience heat shock, which may arise in various organs, including leaves and particularly fruit, when transpiration is insufficient to overcome heat stress. Heat also damages cellular structures, including organelles and cytoskeleton, and impairs membrane function. Heat shock may produce a decrease in overall protein synthesis, accompanied by expression of heat shock proteins, e.g., chaperones, which are involved in refolding proteins denatured by heat.
Heat stress often accompanies conditions of low water availability. Heat itself is seen as an interacting stress and adds to the detrimental effects caused by water deficit conditions. Water evaporation increases along with the rise in daytime temperatures and can result in high transpiration rates and low plant water potentials. High-temperature damage to pollen almost always occurs in conjunction with drought stress, and rarely occurs under well-watered conditions. Combined stress can alter plant metabolism in various ways; therefore understanding the interaction between different stresses may be important for the development of strategies to enhance stress tolerance by genetic manipulation.
Excessive chilling conditions, e.g., low, but above freezing, temperatures affect crops of tropical origins, such as soybean, rice, maize, and cotton. Typical chilling damage includes wilting, necrosis, chlorosis or leakage of ions from cell membranes. The underlying mechanisms of chilling sensitivity are not completely understood yet, but probably involve the level of membrane saturation and other physiological deficiencies. For example, photoinhibition of photosynthesis (disruption of photosynthesis due to high light intensities) often occurs under clear atmospheric conditions subsequent to cold late summer/autumn nights. In addition, chilling may lead to yield losses and lower product quality through the delayed ripening of maize.
Salt and drought stress signal transduction consist of ionic and osmotic homeostasis signaling pathways. The ionic aspect of salt stress is signaled via the SOS pathway where a calcium-responsive SOS3-SOS2 protein kinase complex controls the expression and activity of ion transporters such as SOS1. The osmotic component of salt stress involves complex plant reactions that overlap with drought and/or cold stress responses.
Common aspects of drought, cold and salt stress response [Reviewed in Xiong and Zhu (2002) Plant Cell Environ. 25: 131-139] include: (a) transient changes in the cytoplasmic calcium levels early in the signaling event; (b) signal transduction via mitogen-activated and/or calcium dependent protein kinases (CDPKs) and protein phosphatases; (c) increases in abscisic acid levels in response to stress triggering a subset of responses; (d) inositol phosphates as signal molecules (at least for a subset of the stress responsive transcriptional changes; (e) activation of phospholipases which in turn generates a diverse array of second messenger molecules, some of which might regulate the activity of stress responsive kinases; (f) induction of late embryogenesis abundant (LEA) type genes including the CRT/DRE responsive COR/RD genes; (g) increased levels of antioxidants and compatible osmolytes such as proline and soluble sugars; and (h) accumulation of reactive oxygen species such as superoxide, hydrogen peroxide, and hydroxyl radicals. Abscisic acid biosynthesis is regulated by osmotic stress at multiple steps. Both ABA-dependent and -independent osmotic stress signaling first modify constitutively expressed transcription factors, leading to the expression of early response transcriptional activators, which then activate downstream stress tolerance effector genes.
Several genes which increase tolerance to cold or salt stress can also improve drought stress protection, these include for example, the transcription factor AtCBF/DREB1, OsCDPK7 (Saijo et al. 2000, Plant J. 23: 319-327) or AVP1 (a vacuolar pyrophosphatase-proton pump, Gaxiola et al. 2001, Proc. Natl. Acad. Sci. USA 98: 11444-11449).
Developing stress-tolerant plants is a strategy that has the potential to solve or mediate at least some of these problems. However, traditional plant breeding strategies used to develop new lines of plants that exhibit tolerance to ABS are relatively inefficient since they are tedious, time consuming and of unpredictable outcome. Furthermore, limited germplasm resources for stress tolerance and incompatibility in crosses between distantly related plant species represent significant problems encountered in conventional breeding. Additionally, the cellular processes leading to ABS tolerance are complex in nature and involve multiple mechanisms of cellular adaptation and numerous metabolic pathways.
Genetic engineering efforts, aimed at conferring abiotic stress tolerance to transgenic crops, have been described in various publications [Apse and Blumwald (Curr Opin Biotechnol. 13:146-150, 2002), Quesada et al. (Plant Physiol. 130:951-963, 2002), Holmström et al. (Nature 379: 683-684, 1996), Xu et al. (Plant Physiol 110: 249-257, 1996), Pilon-Smits and Ebskamp (Plant Physiol 107: 125-130, 1995) and Tarczynski et al. (Science 259: 508-510, 1993)].
Various patents and patent applications disclose genes and proteins which can be used for increasing tolerance of plants to abiotic stresses. These include for example, U.S. Pat. Nos. 5,296,462 and 5,356,816 (for increasing tolerance to cold stress); U.S. Pat. No. 6,670,528 (for increasing ABST); U.S. Pat. No. 6,720,477 (for increasing ABST); U.S. application Ser. Nos. 09/938,842 and 10/342,224 (for increasing ABST); U.S. application Ser. No. 10/231,035 (for increasing ABST); WO2004/104162 (for increasing ABST and biomass); WO2007/020638 (for increasing ABST, biomass, vigor and/or yield); WO2007/049275 (for increasing ABST, biomass, vigor and/or yield).
Suboptimal nutrient (macro and micro nutrient) affect plant growth and development through the whole plant life cycle. One of the essential macronutrients for the plant is Nitrogen. Nitrogen is responsible for biosynthesis of amino acids and nucleic acids, prosthetic groups, plant hormones, plant chemical defenses, and the like. Nitrogen is often the rate-limiting element in plant growth and all field crops have a fundamental dependence on inorganic nitrogenous fertilizer. Since fertilizer is rapidly depleted from most soil types, it must be supplied to growing crops two or three times during the growing season. Additional important macronutrients are Phosphorous (P) and Potassium (K), which have a direct correlation to yield and general plant tolerance.
Vegetable or seed oils are the major source of energy and nutrition in human and animal diet. They are also used for the production of industrial products, such as paints, inks and lubricants. In addition, plant oils represent renewable sources of long-chain hydrocarbons which can be used as fuel. Since the currently used fossil fuels are finite resources and are gradually being depleted, fast growing biomass crops may be used as alternative fuels or for energy feedstocks and may reduce the dependence on fossil energy supplies. However, the major bottleneck for increasing consumption of plant oils as bio-fuel is the oil price, which is still higher than fossil fuel. In addition, the production rate of plant oil is limited by the availability of agricultural land and water. Thus, increasing plant oil yields from the same growing area can effectively overcome the shortage in production space and can decrease vegetable oil prices at the same time.
Studies aiming at increasing plant oil yields focus on the identification of genes involved in oil metabolism as well as in genes capable of increasing plant and seed yields in transgenic plants. Genes known to be involved in increasing plant oil yields include those participating in fatty acid synthesis or sequestering such as desaturase [e.g., DELTA6, DELTA12 or acyl-ACP (Ssi2; Arabidopsis Information Resource (TAIR; Hypertext Transfer Protocol://World Wide Web (dot) arabidopsis (dot) org/), TAIR No. AT2G43710)], OleosinA (TAIR No. AT3G01570) or FAD3 (TAIR No. AT2G29980), and various transcription factors and activators such as Lec1 [TAIR No. AT1G21970, Lotan et al. 1998. Cell. 26; 93 (7):1195-205], Lec2 [TAIR No. AT1G28300, Santos Mendoza et al. 2005, FEBS Lett. 579 (21):4666-70], Fus3 (TAIR No. AT3G26790), ABI3 [TAIR No. AT3G24650, Lara et al. 2003. J Biol. Chem. 278 (23): 21003-11] and Wri1 [TAIR No. AT3G54320, Cernac and Benning, 2004. Plant J. 40 (4): 575-85].
Genetic engineering efforts aiming at increasing oil content in plants (e.g., in seeds) include upregulating endoplasmic reticulum (FAD3) and plastidal (FAD7) fatty acid desaturases in potato (Zabrouskov V., et al., 2002; Physiol Plant. 116:172-185); over-expressing the GmDof4 and GmDof11 transcription factors (Wang H W et al., 2007; Plant J. 52:716-29); over-expressing a yeast glycerol-3-phosphate dehydrogenase under the control of a seed-specific promoter (Vigeolas H, et al. 2007, Plant Biotechnol J. 5:431-41; U.S. Pat. Appl. No. 20060168684); using Arabidopsis FAE1 and yeast SLC1-1 genes for improvements in erucic acid and oil content in rapeseed (Katavic V, et al., 2000, Biochem Soc Trans. 28:935-7).
Various patent applications disclose genes and proteins which can increase oil content in plants. These include for example, U.S. Pat. Appl. No. 20080076179 (lipid metabolism protein); U.S. Pat. Appl. No. 20060206961 (the Ypr140w polypeptide); U.S. Pat. Appl. No. 20060174373 [triacylglycerols synthesis enhancing protein (TEP)]; U.S. Pat. Appl. Nos. 20070169219, 20070006345, 20070006346 and 20060195943 (disclose transgenic plants with improved nitrogen use efficiency which can be used for the conversion into fuel or chemical feedstocks); WO2008/122980 (polynucleotides for increasing oil content, growth rate, biomass, yield and/or vigor of a plant).
Cotton and cotton by-products provide raw materials that are used to produce a wealth of consumer-based products in addition to textiles including cotton foodstuffs, livestock feed, fertilizer and paper. The production, marketing, consumption and trade of cotton-based products generate an excess of $100 billion annually in the U.S. alone, making cotton the number one value-added crop.
Even though 90% of cotton's value as a crop resides in the fiber (lint), yield and fiber quality has declined due to general erosion in genetic diversity of cotton varieties, and an increased vulnerability of the crop to environmental conditions.
There are many varieties of cotton plant, from which cotton fibers with a range of characteristics can be obtained and used for various applications. Cotton fibers may be characterized according to a variety of properties, some of which are considered highly desirable within the textile industry for the production of increasingly high quality products and optimal exploitation of modem spinning technologies. Commercially desirable properties include length, length uniformity, fineness, maturity ratio, decreased fuzz fiber production, micronaire, bundle strength, and single fiber strength. Much effort has been put into the improvement of the characteristics of cotton fibers mainly focusing on fiber length and fiber fineness. In particular, there is a great demand for cotton fibers of specific lengths.
A cotton fiber is composed of a single cell that has differentiated from an epidermal cell of the seed coat, developing through four stages, i.e., initiation, elongation, secondary cell wall thickening and maturation stages. More specifically, the elongation of a cotton fiber commences in the epidermal cell of the ovule immediately following flowering, after which the cotton fiber rapidly elongates for approximately 21 days. Fiber elongation is then terminated, and a secondary cell wall is formed and grown through maturation to become a mature cotton fiber.
Several candidate genes which are associated with the elongation, formation, quality and yield of cotton fibers were disclosed in various patent applications such as U.S. Pat. No. 5,880,100 and U.S. patent application Ser. Nos. 08/580,545, 08/867,484 and 09/262,653 (describing genes involved in cotton fiber elongation stage); WO0245485 (improving fiber quality by modulating sucrose synthase); U.S. Pat. No. 6,472,588 and WO0117333 (increasing fiber quality by transformation with a DNA encoding sucrose phosphate synthase); WO9508914 (using a fiber-specific promoter and a coding sequence encoding cotton peroxidase); WO9626639 (using an ovary specific promoter sequence to express plant growth modifying hormones in cotton ovule tissue, for altering fiber quality characteristics such as fiber dimension and strength); U.S. Pat. No. 5,981,834, U.S. Pat. No. 5,597,718, U.S. Pat. No. 5,620,882, U.S. Pat. No. 5,521,708 and U.S. Pat. No. 5,495,070 (coding sequences to alter the fiber characteristics of transgenic fiber producing plants); U.S. patent applications U.S. 2002049999 and U.S. 2003074697 (expressing a gene coding for endoxyloglucan transferase, catalase or peroxidase for improving cotton fiber characteristics); WO 01/40250 (improving cotton fiber quality by modulating transcription factor gene expression); WO 96/40924 (a cotton fiber transcriptional initiation regulatory region associated which is expressed in cotton fiber); EP0834566 (a gene which controls the fiber formation mechanism in cotton plant); WO2005/121364 (improving cotton fiber quality by modulating gene expression); WO2008/075364 (improving fiber quality, yield/biomass/vigor and/or abiotic stress tolerance of plants).